Stop deleting ShortcutsDatabase on the UI thread during shutdown

Deleting ShortcutsDatabase on the UI thread results in destructing sql::Connection.
This results in disk IO and slows down shutdown.

BUG=480822

Review URL: https://codereview.chromium.org/1105843002

Cr-Commit-Position: refs/heads/master@{#327231}

chrome/browser/autocomplete/shortcuts_backend.cc

@@ -146,6 +146,13 @@ void ShortcutsBackend::AddOrUpdateShortcut(const base::string16& text,
 }
 
 ShortcutsBackend::~ShortcutsBackend() {
+  if (db_) {
+    auto* db = db_.get();
+    db->AddRef();
+    db_ = nullptr;
+    if (!BrowserThread::ReleaseSoon(BrowserThread::DB, FROM_HERE, db))
+      db->Release();
+  }
 }
 
 // static